In the recent use of electronic material, demands are continually increasing for heat resistance and surface accuracy in insulating films. When an insulating film is used as a substrate material, and particularly when a metal is sputtered, it becomes possible to form a thinner pattern as surface accuracy increases for the film functioning as a base.
According to JP (Kokai) 4-49026, a liquid-crystal polymer (LCP) film with high surface accuracy can be obtained by the coextrusion of an LCP and a non-LCP. A liquid-crystal polymer film with high surface accuracy can be obtained by using as a lamination film a polyether sulfone (PES) film according to JP (Kokai) 7-323506, and a thermoplastic polyimide (TPI) film according to JP (Kokai) 7-251438.
However, all these reports concern liquid-crystal polymers with boiling points of 285° C.
JP (Kokai) 9-131789 depicts an embodiment in which a liquid-crystal polymer with a melting point of 330° C. is drawn using a PES film as a lamination film, and then the lamination film is peeled off to yield a liquid-crystal polymer.
In an LCP with a melting point of 335° C. or greater, the temperature at which drawing is actually possible is at or greater than the melting point, and drawing at this temperature is impossible with PES, TPI, and the like because of their insufficient strength as lamination films.
Furthermore, JP (Kokai) 9-131789 and 10-34742 have embodiments of manufacturing an LCP film with a melting point of 335° C. or greater, but the surface roughness Rz value of the film is at best 0.5 μm, and its surface accuracy is insufficient.
Thus, none of the reports so far have cited a manufacturing example of a liquid-crystal polymer film with a melting point of 335° C. or greater and a surface roughness Ra of 0.1 μm or less both in the machine direction and in the transverse direction.
An object of the present invention is to provide a liquid-crystal polymer film (and a manufacturing method thereof) with a melting point of 335° C. or greater and high surface accuracy, and specifically with a surface roughness Ra of 0.1 μm or less both in the machine direction and in the transverse direction.
The inventors completed the present invention as a result of intense research aimed at resolving the above-mentioned problems.
Specifically, the present invention provides a liquid-crystal polymer film with a melting point of 335° C. or greater and a surface roughness Ra of 0.1 μm or less both in the machine direction and in the transverse direction, which is obtained by a process in which a laminate of a fluororesin porous film and a resin film made from a liquid-crystal polymer or a polymer alloy containing a liquid-crystal polymer is drawn, and the fluororesin porous film is peeled off.
The present invention also provides a method for manufacturing a liquid-crystal polymer film with a melting point of 335° C. or greater and a surface roughness Ra of 0.1 μm or less both in the machine direction and in the transverse direction, characterized in that a laminate of a fluororesin porous film and a resin film made from a liquid-crystal polymer or a polymer alloy containing a liquid-crystal polymer is drawn, and the fluororesin porous film is peeled off.
The liquid-crystal polymer used in the present invention is preferably a thermotropic liquid-crystal polymer with a melting point of 335° C. or greater, and various types known in conventional practice can be used. A preferable melting point is 335-400° C. Such liquid-crystal polymers include, for example, aromatic polyesters that display liquid crystallinity when melted and are synthesized from monomers such as aromatic diols, aromatic carboxylic acids, and hydroxycarboxylic acids. Typical examples thereof include type 1 polymers comprising p-hydroxybenzoic acid (PHB), terephthalic acid, and biphenyls (formula 1 below); type 2 polymers comprising PHB and 2,6-hydroxynaphthoic acid (formula 2 below); and type 3 polymers comprising PHB, terephthalic acid, and ethylene glycol (formula 3 below). 
In the present invention, it is acceptable to use a polymer alloy containing a liquid-crystal polymer as a polymer alloy component instead of using a liquid-crystal polymer alone. In this case, polymers that can be mixed or chemically bonded with the liquid-crystal polymer include, but are not limited to, polyether ether ketone, polyether sulfone, polyimide, polyetherimide, polyamide, polyamide-imide, polyarylate, and the like. The mixture ratio of the macromolecules and liquid-crystal polymer is preferably (by weight) 10:90-90:10, and more preferably 30:70-70:30. The polymer alloy containing a liquid-crystal polymer also has excellent properties due to the liquid-crystal polymer.
In the present invention, the resin composed of a liquid-crystal polymer or a polymer alloy with a liquid-crystal polymer as the alloy component may also contain additives such as compatibility accelerators, plasticizers, or flame retardants, or fillers such as inorganic powder or fiber, which are added in accordance with the intended purpose.
In the present invention, the liquid-crystal polymer or the polymer alloy containing a liquid-crystal polymer (these will also be referred to hereinbelow simply as “liquid-crystal polymer”) used as a raw film is formed into a film. Film forming in this case can be done by extrusion, roll calendering, or the like. This raw liquid-crystal polymer resin film has a thickness of 5-1000 μm, and preferably 10-500 μm.
The fluororesin porous film used as a lamination film in the present invention preferably has a specific gravity of 1.3 or greater, and more preferably 1.5 or greater, the upper limit of which is commonly about 2.0. The porosity thereof is preferably 40% or less, and more preferably 30% or less, the lower limit of which is commonly about 5%. The elongation at break in the direction of draw is preferably 400% or greater, and more preferably 600% or greater. The upper limit thereof is commonly about 900%.
The fluororesin porous film preferably has an average pore size of 0.05-5.0 μm, and more preferably of 0.2-1.0 μm. The thickness thereof is preferably 5-300 μm, and more preferably 20-150 μm.
In addition to polytetrafluoroethylene, examples of possible fluororesins in the fluororesin porous film include tetrafluoroethylene/hexafluoropropylene copolymers, polyvinyl fluoride, polyvinylidene fluoride, polytrifluorochloroethylene, and the like. In the present invention, an drawn porous polytetrafluoroethylene film is preferred on the basis of heat resistance and chemical resistance.
If the specific gravity of the fluororesin porous film used in the present invention is too small, the surface roughness Ra of the resulting liquid-crystal polymer film increases (the surface becomes rough). The surface roughness Ra of the resulting liquid-crystal polymer film also increases (the surface becomes rough) when elongation at break in the direction of draw is too low.
The method for manufacturing a liquid-crystal polymer of the present invention comprises steps of forming a laminate film, drawing, cooling, and peeling. Each of these steps is described below in detail.
This step entails forming a laminate film by the thermocompression bonding of a fluororesin porous film on both sides of a liquid-crystal polymer film. The preferred temperature for obtaining the laminate film differs depending on the melting point of the liquid-crystal polymer film employed, but it is essentially a temperature which softens at least the surface of the liquid-crystal polymer film, or else the area in contact with the fluororesin porous film or the entire film, while the fluororesin porous film maintains sufficient strength.
When the laminate film is manufactured in this manner, a pair of thermocompression bonding rolls or a hot-press device is used as the thermocompression bonding device. When thermocompression bonding rolls are used, the liquid-crystal polymer film and the two fluororesin porous films laminated to both surfaces thereof are fed to a gap (clearance) between the pair of thermocompression bonding rolls and are thermocompression-bonded in the gap between the thermocompression bonding rolls. The liquid-crystal polymer film used herein can be a solid sheet, a softened film extruded from the T-die of an extruder, or the like. Conversely, when a hot-press device is used, a fluororesin porous film is laid on the bottom plate of the hot-press device, a liquid-crystal polymer film is laid thereon, a fluororesin porous film is placed on top, the resulting assembly is thermocompression-bonded by applying downward pressure with a top plate for a specific period of time, and the product is cooled. In this case, the bottom plate and/or top plate is heated, and at least the surface portion of the liquid-crystal polymer film is softened. The laminate obtained by the laminate film forming step is sent on to the subsequent drawing step either directly or after being cooled.